use selected installer consistently

[orbeckst]

The MDAKit workflow that uses this action seems to fail when selecting mamba as the installer (see MDAnalysis/mdaencore#76 (comment) ).

This may be fixed by using mamba/micromamba instead of conda in

install-mdanalysis/action.yaml

Lines 66 to 67 in 60ef7aa

	conda remove --force mdanalysis --yes || true
	conda remove --force mdanalysistests --yes || true

As a workaround, set installer: conda when using the action.

[orbeckst]

@ojeda-e if you have other examples (potentially) related to the mamba/conda issue please link them here. Thanks.

[ojeda-e]

I'll update this comment as I find them.

I have found two cases so far:

Case 1: when installing MDAnalysis@main

Error

[GH Actions CI/main-tests-6]   ☁  git clone 'https://github.com/codecov/codecov-action' # ref=v5
[GH Actions CI/main-tests-2]   | Resolving Environment                                                                     ✔ Done (9.4 sec)
[GH Actions CI/main-tests-2]   | error    libmamba Could not solve for environment specs
[GH Actions CI/main-tests-2]   |     The following package could not be installed
[GH Actions CI/main-tests-2]   |     └─ mdanalysis =* * does not exist (perhaps a typo or a missing channel).
[GH Actions CI/main-tests-2]   | critical libmamba Could not solve for environment specs
[GH Actions CI/main-tests-2]   ❌  Failure - Main Install with conda [31.73742486s]

Repo affected

• Step Run Cookie CI in MDAKit cookicutter:
  https://github.com/MDAnalysis/cookiecutter-mdakit/actions/runs/25415979007/job/74547423871#step:9:5585

Where I have come across it:

• Feat: Modernize license options cookiecutter-mdakit#156 (comment) -> See Run Cookie CI step error

Case 2: When installing MDAnalysisTests

Error

Reads as

error    libmamba Could not solve for environment specs
    The following package could not be installed
    └─ mdanalysistests =* * does not exist (perhaps a typo or a missing channel).
critical libmamba Could not solve for environment specs
Error: Process completed with exit code 1.

Repo affected

• Step install-deps-tests in MDAKits CI: https://github.com/MDAnalysis/MDAKits/blob/18c99bd8e4f6e15fdf302306d6e9cf0b84c8d2ce/.github/workflows/gh-ci.yaml#L117-L123

Where I have come across it:

• Update membrane curvature metadata MDAKits#372 - > See CI error